This invention relates to systems for creating specific electromagnetic field conditions within specific regions in space, or for focussing electromagnetic energy into dielectric objects with enhanced control.
The ability to create specific electromagnetic field conditions is a core requirement in many medical applications from imaging to therapies. The present invention has applications in both these disciplines, as well as in phased array technology employed for communications and sensing applications.
One application of this invention is the generation of specific field conditions at certain locations in the human body for the purpose of hyperthermia.
The National Cancer Institute of the US National Institutes of Health defines Hyperthermia (also called thermal therapy or thermotherapy) as a type of cancer treatment in which body tissue is exposed to high temperatures (up to 45° C.). Research has shown that high temperatures can damage and kill cancer cells, usually with minimal injury to normal tissues. By killing cancer cells and damaging proteins and structures within cells, hyperthermia may shrink tumors.
This invention is concerned with local hyperthermia in which heat is applied to a small region, such as a tumor. It is possible to use various techniques to deliver energy to heat the tumor. In the context of this invention, either microwave or radio frequencies may be employed to apply the heat. Depending on the tumor location, there are several approaches to local hyperthermia. In the present case, an external approach is employed to treat tumors. The energy is applied by means of an applicator. The applicator is made up of a number of elements that are positioned around or near the appropriate region, and energy is focused on the tumor to raise its temperature using phased array techniques.
Hyperthermia is often applied in combination with other therapies such as radiation therapy and/or chemotherapy. Hyperthermia has been performed as part of the treatment of many types of cancer, including sarcoma, melanoma, and cancers of the head and neck, brain, lung, esophagus, breast, bladder, rectum, liver, appendix, cervix, and peritoneal lining (mesothelioma).
A phased array antenna is an antenna made up from a number of small(er) radiating elements, each with its own feed point. Phased array antennas are electrically steerable, which means the physical antenna can be stationary yet the antenna pattern can be manipulated by adjusting the amplitude weighting and phases of each element such that it is focused towards a particular region or such that it enables location of objects in space. Phased arrays can also be utilized to generate specific field conditions at certain locations in space or to focus radio frequency (RF) energy into dielectric objects in order to elevate the temperature of a target region inside the dielectric object or patient or induce fields and currents in a patient to excite atoms, nerves or other cellular mechanisms.
A phased array can be used for hyperthermia by focusing RF energy into the patient such that the temperature is elevated. When a phased array is used for this purpose, it is termed an applicator as it applies energy to the patient. The phased array or applicator elements are fed by a multi-channel RF or microwave power source where the phase and amplitude signals are agile such that the RF or microwave energy can be focused in a target region or tumor. The number of array elements and placement of these elements with respect to the target region define the quality of the focus that can be achieved.
The exemplar of RF hyperthermia will be used to illustrate the benefits of the invention. Although many systems have been proposed and used in the past for hyperthermia treatment of tumors, either alone or in conjunction with other therapies, the consistency and quality of the treatment has generally been lacking. Of utmost importance in local hyperthermia is the ability to apply or focus the energy from the applicator into the target region, tissue or tumor. To achieve satisfactory treatment outcomes, the whole target region should be heated sufficiently. To ensure this, a good electromagnetic applicator and patient specific models are most preferably used to plan and optimize the treatment. This step of accurately predicting the deposition of energy (and/or temperature rises) and optimizing such for best tumor treatment has been lacking in hyperthermia systems and has contributed to poor outcomes. During the treatment itself, in which RF or microwave power is applied to the hyperthermia array with the excitation amplitudes and phases as determined from the treatment plan, it is essential from a quality assurance point of view that the electromagnetic fields generated by each element is monitored to determine that the correct planned treatment is actually being applied.
Common to all phased array antennas or hyperthermia applicators, is the requirement for a multichannel source which can generate powerful signals with accurately controllable amplitude and phase with which to feed the individual electromagnetic field generating elements. It is not important for this invention which method is used to generate these signals.
Multi-element or phased array applicators generally dispose the elements of the array around the patient with a water bolus filling the space between patient and array to provide surface cooling and lower reflections at the patient interface. U.S. Pat. Nos. 4,672,980, 5,251,645 and 5,441,532 all show typical phased array applicators. Each has the elements disposed in a circular array around the patient with the individual antenna elements (or pairs of elements in U.S. Pat. No. 4,672,980) excited by an RF power source with controlled amplitude and phase. None of these systems measure the actual applied signals or any power reflected which would reduce the effective radiated power. These factors therefore increase the uncertainty, In U.S. Pat. Nos. 5,251,645 and 5,441,532, field sensors are placed in and around the body of the patient to measure the overall applied field at those points and claims that using the values from these sensors the array excitation can be controlled such that the energy is focused into the target. U.S. Pat. No. 4,672,980 uses a different approach where temperature measurement catheters are inserted into the patient and the system controlled to maximize the temperature increase in the target region. The draw-back of both approaches is that the human body is highly inhomogeneous and there is no intuitive relationship between applied excitations of the array and the energy deposition pattern. In essence these approaches assume that knowing the field or temperature at a few points is a substitute for knowing the radiation from each array element.
In the literature, Paulides et al 2007 describe a typical state of the art system, where the magnitude and phase of the applied signals to each applicator element is measured along with the reflected power, such that the control values can be adjusted such that the applied signals in light of reflections are as desired. When used with proper treatment planning this system has the potential to perform satisfactorily.
However, the system relies on a computer simulation model fully defining the actual device and no means is available to fully account for changes in registration of the patient with respect to the applicator for the element impedance and mutual coupling element of the excitation.
In the broader context of phased arrays for other applications, U.S. Pat. No. 5,867,123 uses a technique of exciting single elements and observing the signals received by adjacent elements for built-in testing and failure analysis. Fulton and Chappell, 2009, review different calibration techniques for phased arrays and states arrays should be calibrated in an anechoic environment to determine the coupling matrix to enable compensation of the mutual coupling in the array. Additionally, it is noted that internal electronic hardware can be introduced for the monitoring of any changes from the initial calibrated coupling or transmit chain gains allowing correction to be applied. Lee et al, 1992/3, introduced a transmission (microstrip) line into the antenna panel to couple with each element so that transmit and receive functions of the electronics could be tested. The transmission line receives energy from all elements or injects energy into all elements of the array simultaneously.